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Stage 18 embryos have a greatest length of 13 to 17 mm and an estimated postfertilization age of approximately 44 days. In the head region the tip of the nose becomes discernable as well as the volmeronasal organ, eyelids, and nasolacrimal ducts. The auricular hillocks begin to blend to form parts of the definitive external ear. Two or three semicircular ducts form from the otic vesicle in the otic capsule. The subarachnoid space surrounds the entire central nervous system and the choroid plexus is forming in the roof of the 4th ventricle.


New structures include brachiocephalic artery, clavicle, femoral nerve, intervertebral discs, pyloric antrum of stomach, scapula, suprarenal gland cortex.

Section 437

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